Generally, natural gas is transported in a gaseous state via onshore or offshore gas pipelines, or is transported to a distant destination by an LNG carrier after being liquefied into LNG.
LNG is obtained by cooling natural gas to cryogenic temperatures, for example, about −163° C. and has a volume of about 1/600 that of natural gas in a gaseous state. Thus, LNG is suited to long distance transport by sea.
An LNG carrier, which is designed to carry LNG by sea to an onshore source of demand, or an LNG regasification vessel (LNG RV), which is designed to carry LNG by sea to an onshore source of demand, regasify the LNG, and discharge the regasified LNG to the onshore source of demand, is provided with a storage tank capable of withstanding cryogenic temperatures of LNG (commonly referred to as “cargo hold”).
Recently, there is increasing demand for floating offshore structures such as LNG-floating production, storage and offloadings (FPSOs) and LNG-floating storage and regasification units (FSRUs). Such a floating offshore structure is also provided with a storage tank that is used in LNG carriers or LNG RVs.
An LNG-FPSO is a floating offshore structure that is designed to liquefy produced natural gas, store the liquefied natural gas in a storage tank, and, if necessary, offload the LNG onto an LNG carrier.
An LNG-FSRU is a floating offshore structure that is designed to store LNG offloaded from an LNG carrier in a storage tank and, if necessary, regasify the LNG and supply the regasified LNG to an onshore source of demand.
Such an offshore vessel carrying LNG by sea or storing LNG, such as LNG carriers, LNG RVs, LNG FPSOs, and LNG FSRUs, is provided therein with a storage tank storing LNG in a cryogenic state.
Such a storage tank is divided into an independent storage tank and a membrane-type storage tank depending on whether an insulator thereof directly receives a load of a cargo.
In addition, the membrane-type storage tank is divided into a GTT NO 96-type tank and a TGZ Mark III-type, and the independent storage tank is divided into an MOSS-type tank and an IHI-SPB-type tank.
Here, the insulation material and structure of the membrane-type storage tank may vary depending upon the type of a special metal sheet that is used as a material for the storage tank. Specifically, the GTT NO 96-type tank is manufactured using an Invar sheet (an alloy mainly composed of iron and nickel and having a very low coefficient of thermal expansion) and the Mark III-type tank is manufactured using a stainless steel sheet.
The GTT NO 96-type storage tank has a structure in which a primary and secondary sealing wall formed of an Invar sheet having a thickness of 0.5 mm to 1.5 mm and a primary and secondary insulation wall formed of a plywood box and perlite are alternately stacked on an inner wall of a hull.
In the GTT NO 96-type storage tank, the secondary sealing wall has almost the same level of liquid tightness and strength as the primary sealing wall, thereby safely supporting a cargo for a considerable period of time even when the primary sealing wall leaks.
An insulation system of the GTT NO 96-type storage tank is composed of two layers of insulation boxes formed of Invar (36% nickel), pearlite, and plywood.
Now, a typical cargo hold insulation structure for LNG carriers will be described with reference to the drawings.
FIG. 1 is a perspective view of a typical cargo hold insulation structure for LNG carriers.
Referring to FIG. 1, the typical cargo hold insulation structure for LNG carriers includes a plurality of insulation panel assembly units 1 disposed in series, wherein each of the insulation assembly units includes a lower insulation panel 10, an upper insulation panel 20, a flat joint 30, a top bridge panel 40, and a membrane sheet 50.
The lower insulation panel 10 is secured to an inner wall of a storage tank 2 (or inner hull) using epoxy mastic 3 and a stud bolt 11.
The flat joint 30 is disposed in a space between the lower insulation panels 10 of the respective insulation panel assembly units 1 facing each other to seal the space and provide secondary insulation.
The lower insulation panel 10 may be formed of reinforced-polyurethane foam and is provided on an upper surface thereof with a rigid triplex 12 (or rigid secondary barrier (RSB). In other words, the lower insulation panel is provided with plywood on a surface thereof facing the inner wall 2 of the tank and is provided with the rigid triplex 12 on the other surface (i.e., upper surface) thereof.
The upper insulation panel 20 includes a sawing line 21, a securing base support 22 (or metallic insert), an anchor strip 23, and a thermal protection 24 and is attached to the upper side of the lower insulation panel 10.
The top bridge panel 40 is disposed in a space between the upper insulation panels 20 of the respective insulation panel assembly units 1 facing each other to seal the space and provide primary insulation.
The upper insulation panel 20 may be formed of reinforced polyurethane foam and may be provided on an upper surface thereof with plywood.
The sawing line 21 is formed in the upper insulation panel 20 to prevent deformation of a hull due to contraction and expansion at cryogenic temperatures and may include a plurality of transverse and longitudinal sawing lines crossing at right angles to form a grid pattern.
The thermal protection 24 is disposed at at least one end of the anchor strip 23 to compensate for reduction in resistance of the lower and upper insulation panels 10, 20 to damage by deformation of the hull and thermal deformation of the membrane sheet 50.
A gap 41 is formed between the upper insulation panel 20 and the top bridge panel 40.
The securing base support 22 includes a plurality of securing base supports formed in the upper insulation panel 20.
The anchor strip 23 is formed of stainless steel and is secured to the upper insulation panel 20 using a rivet R.
The thermal protection 24 serves to prevent the membrane sheet 50 from being directly welded to the upper insulation panel 20 while preventing the upper insulation panel 20 from being damaged by flame or heat generated during welding of the membrane sheet 50.
The flat joint 30 is disposed in a space between the lower insulation panels 10 of the respective insulation panel assembly units 1 facing each other to provide secondary insulation. The flat joint 30 may be formed of glass wool.
The top bridge panel 40 is attached to upper sides of the flat joint 30 and the lower insulation panel 10 without the attached upper insulation panel 20 to seal a space between the upper insulation panels 20 of the respective insulation panel assembly units 1 facing each other and to provide primary insulation.
The top bridge panel 40 may be formed of reinforced polyurethane foam and may be attached to an upper side of a flexible triplex 13 disposed on the lower insulation panel 10 and the flat joint 30.
The top bridge panel 40 is disposed such that a gap 41 is formed between the top bridge panel and each of the upper insulation panels 20 of the respective insulation panel assembly units 1 facing each other, thereby preventing the lower and upper insulation panels 10, 20 from being damaged by deformation of the hull and thermal deformation of the membrane sheet 50, along with the sawing line 21.
The membrane sheet 50 is securely coupled to the upper sides of the upper insulation panel 20 and the top bridge panel 40 through the anchor strip 23.
The membrane sheet 50 is a corrugated membrane sheet and may be embossed to have uneven upper and lower surfaces.
Since an LNG carrier is intended to carry LNG at cryogenic temperatures, for example, about −163° C., by sea, various advanced technologies are required to provide heat insulation performance, structural performance, hermeticity and the like to a cargo hold of the LNG carrier. Particularly, for a membrane-type cargo hold for LNG carriers, a membrane sheet is welded to an upper side of an upper insulation panel to prevent leakage of LNG.
In a typical cargo hold insulation structure for LNG carriers, in order to provide hermeticity to the cargo hold, individual membrane sheets 50 are secured to an anchor strip 23 of an upper insulation panel 12 by spot welding, followed by line welding of adjacent overlapping membrane sheets 50.
Thus, such a typical anchor strip serves to allow the membrane sheet to be spot-welded thereto while preventing damage to the upper insulation panel due to flame or heat generated during welding.
However, the typical anchor strip is formed of SUS and thus requires additional components such as a securing rivet and additional processes such as machining of rivet mounting holes in both the anchor strip and the upper insulation panel and riveting, causing increase in production cost and product price.